I have used my laptop/ipad/cell phone plenty of times while parked at the gate (and while the door was open).

Laptops are only off limits after landing, and I assume to keep the space around the seats clear while the aircraft is still moving and to prevent people from getting up to retrieve it.

I have even had the pilot or FA announce that it is OK to use electronic devices while parked, with door closed, waiting for a ground delay (e.g. if you are parked close to the runway waiting for a while, not while in the stop-and-go line)